There have been two general approaches to the design of systems that combine air conditioning and dehumidification. In one approach, the air is cooled down to the dew point for the desired final moisture content, in order to remove all undesired moisture. Because this dew point is generally below the desired final air temperature, the air at this dew point must then be heated, usually by mixing it with air that is already in the room being air conditioned.
In the second approach, the air is dehumidified, before cooling it, by exposing it to a desiccant, which absorbs moisture from the air. If the absorption of moisture by the desiccant takes place without any heat flow, i.e. at constant enthalpy, then the air will become hotter as it is dehumidified. The air must then be cooled even more than if it had remained at the outside temperature, which also lowers the efficiency of the air conditioner.
For this reason, it is generally considered to be desirable to remove heat from the desiccant while it is absorbing moisture. In a dehumidifier described in U.S. Pat. No. 6,018,954, a heat pump is used to remove heat from the desiccant while it is absorbing moisture, and to transfer the heat to a regenerator where the desiccant gives off moisture so that it can be used again.
In any dehumidifier using a desiccant, it is desirable to expose a large surface area of desiccant to the incoming air, in order to maximize the throughput of air that can be dehumidified. In the case of liquid desiccant, a large surface area has been produced by spraying small droplets of desiccant through the air, and by dripping the desiccant onto a sponge. Because the desiccant quickly becomes saturated with moisture if it has a large surface area, in all of these dehumidifiers there is a reservoir of desiccant which does not have a large surface area. When the dehumidifier is operating, the desiccant is continuously drawn from the reservoir, a small quantity of the desiccant is exposed to the air in a form with a large surface area (droplets, or sponge, for example) and it is then returned to the reservoir. Dehumidifiers using a reservoir in this way are described in U.S. Pat. No. 6,018,954, and in PCT patent publications WO 99/26025 and WO 99/26026, the disclosures of which are incorporated herein by reference. On a slower time scale, as the absorbed moisture builds up in the reservoir, the desiccant in the dehumidifier reservoir is circulated into a regenerator, where the moisture is removed from the desiccant by heating it, and the desiccant is circulated back into the dehumidifier reservoir. Within the regenerator, desiccant is also drawn from a reservoir and a small quantity is exposed to the air in a form with a large surface area, for example droplets or a sponge, to speed up the rate at which the moisture is removed from the desiccant.
Modern room air conditioners generally use mechanical heat pumps to cool the air, which requires a large power input. A more energy efficient method of cooling air, known since ancient times, is to use a fan to blow the air past evaporating water. This method consumes much less energy than a heat pump, because it makes use of the free energy inherent in ambient air at less than 100% humidity. By comparison with this free energy, the energy consumed by the fan is trivial. The disadvantages of evaporative cooling are that the air that is cooled has a higher moisture content than the ambient air, and the method doesn't work well at high ambient humidity. Nevertheless, evaporative cooling is often used to cool open environments, for example stockyards, where low humidity is not so important, ambient humidity is not too high, and low power consumption is important. Evaporation of water has also been used to cool the refrigerant in heat pumps used in many cooling systems, and (as described in U.S. Pat. No. 6,018,954) to cool the refrigerant used in a heat pump used for cooling the desiccant in a dehumidifier and heating the desiccant in the associated regenerator.